Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)

[Dmitry Torokhov]

On Mon, Dec 12, 2011 at 03:50:11PM -0800, Linus Torvalds wrote:
> On Mon, Dec 12, 2011 at 10:02 AM, Dmitry Torokhov <dtor@xxxxxxxxxx> wrote:
> > From: Andrei Warkentin <andreiw@xxxxxxxxxx>
> >
> > 1 is a power of two, therefore rounddown_pow_of_two(1) should return 1.
> > It does in case the argument is a variable but in case it's a constant
> > it behaves incorrectly and returns 0. Probably nobody ever did it so
> > this was never noticed, however drivers/net/vmxnet3 with latest GCC does
> > and breaks on unicpu systems.
> >
> > This is similar to Rolf's patch to roundup_pow_of_two(1).
> 
> Umm. I already applied this patch, but then I started looking at it
> more, and asked myself:
> 
>  - Why is that "n == 1" test there AT ALL?
> 
> Afaik, that whole test is just plain stupid. It seems to have been
> copied from the "roundup()" case (where it exists due to the "-1/+1"
> hackery that breaks ilog2()) without any thought about the actual math
> of the function at all.
> 
> I think the *real* fix is to just remove that incorrect line, no?

Yes, you are right, special-casing for 1 is not necessary in rounddown
case.

Thanks,
Dmitry

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